Output bias on Marshall Valvestate

[Ewan Penkey]

Hi all,
I have a Marshall Valvestate 8080 with some crossover distortion on the output.
There's no bias pot I can see.
Transistor amps are not my strongest suit.
Can anyone point me at which resistor(s) value I should be checking / adjusting please?

Many thanks

Ewan

[g1]

Have the output devices been changed? If not, I would want to be sure it is not a fault, in which case bias adjustment would just be a band-aid solution.
A trim-pot for idle current would usually be in the R73 postion, so that would probably be the one to tweak. Lowering R73 value should make the outputs run hotter, so you can try just tacking a parallel resistor across R73. Start with small increments, like 6K8 in parallel to create approx. 620R value for R73.
Measuring DC mV across R96 or R118 will tell you the idle current of the output transistors (V/R to solve for current).

[Jon Snell]

Have the output devices been changed? If not, I would want to be sure it is not a fault, in which case bias adjustment would just be a band-aid solution.
A trim-pot for idle current would usually be in the R73 postion, so that would probably be the one to tweak. Lowering R73 value should make the outputs run hotter, so you can try just tacking a parallel resistor across R73. Start with small increments, like 6K8 in parallel to create approx. 620R value for R73.
Measuring DC mV across R96 or R118 will tell you the idle current of the output transistors (V/R to solve for current).

Monitor the voltage across TR9 Collector to Emitter, if less than 1volt, suspect the transistor if the resistors on the base are correct value.
Test condition; No signal, no load.

[Ewan Penkey]

Thanks very much both for your replies.

All the component values checked out ok.
I clipped a 5K pot across R73 and tweaked it until I had 2.5mV drop across an emitter resistor.
When I put some signal in, the crossover distortion was gone. Tweaking back and forth a bit, the same setting was the equivalent of the distortion just disappearing, plus a tad.
I ended up fitting a 4k7 across R73, and all is well, and I've learned a little bit more.

Thanks again.